1. Field of the Invention
This invention relates to telemetry systems and methods for receiving data from remote sensors, particularly those systems that receive data from a large array of sensors located in an unprotected environment or an area in which electrical power is unavailable.
2. Description of the Related Art
Numerous sensors produce electrical signals that vary in accordance with some external stimulus, such as sound, light, pressure, or any other physical phenomenon. Often, such a sensor has a low level output that requires some kind of conditioning, such as filtering or amplification, prior to being sent to a receiver. This is often accomplished with electronic signal conditioning circuits, necessarily located close to the sensor in order to minimize the losses and distortion that can occur when a low level output has to travel a significant distance by wire to reach a receiver.
Problems can arise in such an all-electronic system when the sensor is located in an unprotected environment, such as underwater or in space, and the sensor signal is to be received in a stable, protected environment, such as inside a submarine or a spacecraft. If conditioning electronics are required, these circuits, due to their close proximity to the sensor, are subjected to the same environment as the sensor. Furthermore, in many applications electrical power is unavailable or undesirable in the unprotected environment, making use of an all-electronic telemetry system difficult or impossible. If the system includes a large array of sensors located in an unprotected environment, the possible problems can be much greater. A large number of cables must now penetrate the boundary between the unprotected environment and the protected one, causing difficulty if the protected environment must be isolated from the unprotected one, as on a submarine. Multiplexing circuits can be added to minimize the number of boundary penetrations, but they would also need to be located in the unprotected environment of the sensors.
These "outboard" electronic components, located in the unprotected environment near the sensors, such as amplifiers, multiplexers, and filters, are complex electronic devices that require periodic maintenance and repair. This maintenance can be difficult or impossible due to the inaccessibility of the components and the harshness of the environment, thus raising serious system reliability concerns. Furthermore, particularly for a large array of sensors, severe weight and cost penalties can be incurred when conditioning electronics are needed for each sensor, and these conditioners must be designed to tolerate the harshness of the environment. Inter-channel crosstalk and susceptibility to electromagnetic interference are other problems inherent in all-electronic telemetry systems. Finally, as mentioned above, the unavailability of electrical power may make an electronic system infeasible.